the
headstock is a part, and the locomotive is an organization of which the
engine is an element. But the headstock subclass is not superior
necessarily to the tailstock subclass simply because the headstock is
commonly more complex than the tailstock. Yet arbitrary preference for
classification in the headstock subclass may be established by position
where an application or a patent contains claims for both.

Thus in a class that is founded on a well-chosen basis that brings
together things bearing close resemblances to each other, all types that
contain the elements essential to produce a complete practically
operative means will be found in subclasses that have a position
somewhere between the beginning and end of the list of subclasses of the
class. Those that add features of elaboration of the essential types and
those that are highly specialized to some particular purpose within the
definition of the class will stand above the essential type subclasses,
while those subclasses for parts and details will stand below those for
the essential types.

_Indented schedules._--In an indented schedule all subclasses in the
first column reading from the left are species to the genus represented
by the class title.[9] All subclasses indented under another subclass
are species to the genus represented by the subclass under which they
are indented. If a title has no number, it represents merely a
subject-matter to be divided, a genus,--having no representatives except
in the species under it. If a subclass having a generic title has a
number, it not only represents a subject-matter to be divided into
species but also all other species not falling within the titles
indented. Although these relative positions might imply that only
proximate species are indented one place, yet mechanical difficulties
render it impracticable to so arrange that all species shall be indented
under their proximate genera.

Indention properly carried out has a tendency to prevent in the process
of logical division the logical fault of proceeding from a high or broad
genus to a low or narrow species. This latter fault may inadvertently
separate things that belong together. If, for example, it were desired
to divide balls in the stated illustration according to material, an
immediate division of balls into aluminum, zinc, glass, ivory, rubber,
would be less useful than to divide into mineral materials and
nonmineral materials as follows:

Balls--